Preplating treatment for maraging steels



United States Patent 3,505,095 PREPLATING TREATMENT FOR MARAGING STEELSGeorge S. Petit and Ralph R. Wright, Oak Ridge, Tenn., assignors to theUnited States of America as represented by the United States AtomicEnergy Commission No Drawing. Filed Apr. 5, 1967, Ser. No. 628,790 Int.Cl. B44d 1/092 US. Cl. 117-50 4 Claims ABSTRACT OF THE DISCLOSURE Thecoating of maraging steels, i.e., low carbon steels containing about 17to 25 weight percent nickel, 3 to 5 weight percent molybdenum, and smallpercentages of cobalt, aluminum, and titanium, with a protective metalplate has been hampered by the formation of a layer of black smut on thesteel surfaces during preplating pickling operations. This smut isbelieved to be due to the presence of a molybdenum compound on the steelsurfaces that is removed by contacting the affected surfaces with anaqueous solution of lactic acid and hydrogen peroxide.

The present invention relates generally to treating low carbon maragingsteels for reception of protective plating, and more particularly to thepreparation of the surfaces of such steels by removing a layer of blacksmut which interferes with the adhesion of nickel plating with thesteels. This invention was made in the course of, or under, a contractwith the US. Atomic Energy Commission.

It has been recognized for some time that maraging steels possess uniqueproperties which render the use of such steels particularly desirable inapplications where strength and toughness are of major importance, suchas, for example, in the fabrication of liquid centrifuge rotors orbowls, bearings, shafts, tubulations, seals, etc. Maraging steel is aniron-nickel alloy composed primarily of 17 to 25 weight percent (w/o)nickel, about 3 to 5 w/o molybdenum, about 6 to 9 w/o cobalt, less thanabout 0.03 w/o carbon, fraction percentages of titanium and aluminum,and the balance essentially iron. This type steel is a high-strengthalloy possessing good ductility, exceptional toughness, and develops amartensitic structure and its high-strength properties by annealing atelevated temperatures, e.g., about 500 C. for a period of several hours.

While maraging steel possesses the desired properties of high strengthand toughness, it is subject to wear and also to corrosion by solutionssuch as cesium chloride which may be employed in centrifuge operations.This shortcoming of maraging steel is readily overcome by providing thesteel with a protective coating of metal such as nickel, which may beapplied by electroplating or by non-electrolytic means.

Normally, the procedure for applying a protective metal coating, e.g., anickel plate, to a base metal requires that the surface of the latter befirst conditioned in such a manner that the nickel plate tightly bondsto the base metal when deposited by an electroplating operation or by anelectroless plating bath. This surface preparation is somewhatconventional and includes the removal of gross contaminants such asrust, oxide, scale, grease, and the like by employing. solvents orscouring powders for the removal of the grease and acid pickling bathsfor the removal of the other contaminants. In addition to the.

removal of the contaminants, the acid pickling bath also etches orslightly roughens the surface of the base metal to make the base metalmore likely to provide a good bond with the nickel plate. However, itwas found that 3,505,095 Patented Apr. 7, 1970 ice maraging steel, whenheat treated prior to final machining to give the metal its desirablestrength properties, was not receptive to nickel plating in that thenickel plating was either prevented from depositing on the maragingsteel or, if deposited, did not adequately bond with the steel. Thispeculiar trait of maraging steel was found to be due to the formation ofa velvety layer of a black smut-like substance on the steel surfaceduring the acid pickling operation. It is believed that this smut is amolybdenum compound, perhaps a form of molybdenum oxide, which forms onthe surface of the maraging steel when heat treated possibly because theheat treating step causes molybdenum to migrate to the surface.

Efforts to remove or otherwise compensate for the smut formation havenot been entirely successful. For example, the modification ofconventional pretreating baths by pickling with a chloride-acidactivator, e.g., a hydrochloric acid solution, has not providedsatisfactory results. Another unsuccessful effort to improve the bondingof the nickel plating involved the use of an electro-strike coating in anickel chloride-hydrochloric acid solution. It has been suggested thatan anodic-type pre treatment using a bath of sulfuric acid with a smalladdition of hydrofluoric acid would provide a solution to the smutproblem. However, the use of hydrofluoric acid would be objectionablesince the surface of the maraging steel article would be attackedexcessively so as to render it extremely diflicult to provide thesurface finish necessa-ry for applications requiring smooth surfaces.Further, with maraging steel articles of relatively complex shapes inthat configurations such as corners, threads, surface protuberances,sharp angles, and the like may be present, the use of anodic treatmentsis not desirable since the high current densities encounteredin suchareas may produce excessively large diametral changes in the article soas to render it unacceptable for its intended purpose. Anotherdisadvantage to using anodic treatments is due to the fact that suchtreatments require that the cathode have a configuration similar to thatof the steel article being treated. Also, such anodic treatments aresomewhat time-consuming and relatively expensive.

It is, therefore, the aim of the present invention to minimize orobviate the above and other shortcomings or drawbacks previouslysuffered in providing maraging steels with a protective metal coatingsuch as nickel. This goal is achieved by contacting maraging steelarticles with an aqueous solution of lactic acid and hydrogen peroxide.This treatment prepares the article surface for reception of an adherentmetal coating by converting the smut to a compound which is readilydissolved in the solution.

An object of the present invention is to render maraging steel surfacesacceptable for receiving an adherent coating of nickel or anothersuitable metal.

Another object of the present invention is to provide a treatment formaraging steel surfaces which is applicable after the conventionalcontaminant removal to produce a surface free of smut for facilitatingthe application and bonding of nickel coatings.

Generally, the present invention involves the employment of a uniquesurface treatment for maraging steel that has been found to be capableof removing or otherwise rendering ineffective a layer of a molybdenumcompound which heretofore prevented metal coatings from tenaciouslyadhering with the steel. As briefly mentioned above, the molybdenumcompound (composition unknown) is believed to be formed on the surfaceof maraging steel articles during the heat treatment operation requiredto provide the steel with its desired strength properties. Thismolybdenum compound appears on the surface of the maraging steel as auniform and continuous layer or coating and has the appearance of asome- 3 what velvet-like black smut after the steel has been subjectedto acid pickling treatments.

The above-mentioned unique treatment consists of an aqueous solution orbath of lactic acid and hydrogen peroxide that de-smuts the exposedsurface of the maraging steel by oxidizing the molybdenum compound to ahigher valence state and then converting this compound to a complex ofan unknown composition that is readily dissolved in the aqueoussolution. This smut removing solution preferably consists of about 10 to20 percent (volume) lactic acid, about 5 to percent (volume) of30-percent hydrogen peroxide, and the balance water. The smut removal isusually accomplished by dipping or immersing the smut carrying maragingsteel article into the treating solution after completing theconventional degreasing and pickling treatments for a duration of about2 minutes with the solution at room temperature. Upon completing thede-smutting operation the steel article is preferably thoroughly rinsedin distilled or de-ionized water to assure minimal transfer ofimpurities into the plating bath.

By utilizing the de-smutting treatment of the present invention togetherwith the conventional degreasing and pickling treatments the maragingsteel articles are provided with a clean surface to which nickel andother metal coatings tightly bond. The particular procedure for applyingthe metal coating is not critical in that conventional electrolessplating baths or electroplating procedures may be readily employed.Further, while this invention relates primarily to the coating ofmaraging steel with nickel, it is to be understood that coatings ofother metals such as copper, silver, gold, etc., may be adequatelybonded to maraging steel by practicing the present invention.

Inasmuch as a highly uniform plating is desirable for applications suchas centrifuge rotors, it may be desirable to use a pickling treatmentwhich is capable of providing minimal surface roughness on the maragingsteel articles. For example, a machined article having a surfaceroughness of approximately 8 microinches exhibits a surface roughness ofabout 13 microinches after cleaning and plating when the picklingsolution is composed of 400 grams of Al(NO -9H O and 400 milliliters 38%HCl diluted to one liter with Water. Pickling in this solution may beachieved in about 2 to 5 minutes with the solution at a temperature of42 C. On the other hand, where such surface uniformity is not necessary,a cleaning and pickling solution of concentrated (38%) HCl with about 5percent (volume) of 30% hydrogen peroxide may be employed. Platedarticles using this treatment together with a degreaser and thede-smutting solution of this invention have a surface roughness of about40 to 80 microinches.

In order to provide a better understanding of the invention, an exampleof a typical preplating treatment of a maraging steel article is setforth below. This example is merely illustrative and is not intended tolimit the invention to the particular degreasing and pickling agents,plating material, or the sequence and duration of the various treatingsteps.

A centrifuge rotor of maraging steel heat treated and machined to thedesired dimensions is first scrubbed with a commercially availablescouring powder to remove some of the gross contaminants and washedthoroughly with tap Water. The rotor is then vapor degreased with.trichloroethylene and thereafter immersed for a period of 4 minutes ina pickling solution consisting essentially of 400 grams of Al(NO -9H Oand 400 milliliters of 38-percent HCl diluted to one liter with water.The solution is maintained at 42 C. After completing the picklingtreatment the rotor is rinsed with tap water and then immersed for aperiod of 2 minutes in the de-smutting solution consisting essentiallyof 15 percent lactic acid, 8 percent hydrogen peroxide, and the balanceWater. The de-smutting solution is preferably maintained at roomtemperature. The rotor is then rinsed in distilled water to complete thepreparation necessary to receive and bond with a nickel plate of desiredthickness that may be subsequently applied in any desired manner.

It will be seen that the present invention sets forth a significantimprovement in the art for providing maraging steels with protectivemetal plating or coatings in that the unique cleaning treatment of thisinvention assures that the bond of the plate to the surface of themaraging steel is not affected by the molybdenum compound (smut)heretofore found highly deleterious to the bond. Another advantage isthat maraging steels can be provided with soundly bonded metal coatingswithout concern as to whether or not the maraging steel had beenpreviously heat treated. Further, while the invention is primarilyconcerned with the treatment of maraging steels, it is to be understoodthat the other steels and alloys which suffer from the presence of themolybdenum compound on their surfaces may be similarly treated andplated with satisfactory results. Additionally, maraging steels andother metals may have some of the undesirable molybdenum compound onsurfaces thereof regardless of whether or not the metal has beensubjected to heat treatments. Consequently, the surface treatment of thepresent invention is also applicable to non-heat treated metals, sincethe removal of even trace amounts of the bond-resisting compound fromthe metal surface results in the establishment of a better bond betweenthe coating material and the base metal.

What is claimed is:

1. The method of improving the bonding properties of protective metalcoatings to a molybdenum containing metal having surfaces thereof coatedwith a layer of a bond-resisting molybdenum compound, comprising thetreating of the molybdenum containing metal prior to the application ofthe metal coating by contacting said surfaces thereof with an aqueoussolution containing as essential ingredients lactic acid and hydrogenperoxide.

2. The method claimed in claim 1, wherein the solution consistsessentially of about 10 to 20 volume percent lacticacid, about 5 to 10volume percent of 30-percent hydrogen peroxide, and the balance water.

3. The method claimed in claim 2, wherein the metal is contacted withthe solution for a period of about 2 minutes.

4. The method claimed in claim 3, wherein the metal is a steel alloyconsisting essentially of 17 to 25 weight percent nickel, about 3 to 5weight percent molybdenum, about 6 to 9 weight percent cobalt, less thanabout 0.3 weight percent carbon, fractional weight percentages oftitanium and aluminum, and the balance iron.

References Cited UNITED STATES PATENTS 3,029,158 4/1962 Lee et al 1l7-5O3,147,154 9/1964 Cole et al. ll7l3() XR 3,386,896 6/1968 Finne 1175O XROTHER REFERENCES Downey, T. A., Organic Acids in Metal Cleaning andFinishing, In Metal Finishing, vol. 60, No. 10, October 1962, pages50-57 and 65.

ALFRED L. LEAVITT, Primary Examiner JANYCE A. BELL, Assistant ExaminerUS. Cl. X.R.

